RFR: 8228363: ContextMenu.show with side=TOP does not work the first time in the presence of CSS [v4]

[Robert Lichtenberger]

> By using the anchor location facility of PopupWindows we can avoid miscalculation of the
> menu's height entirely.
> This fix also cleans up some documentation issues.
> This fix introduces tests that check the correct positioning (test_position_)
> test_position_withCSS reproduces the problem that is fixed with this patch.
> The other test_position_ cases serve as "proof" that no regressions are introduces.
> They work before and after the fix is introduced.

Robert Lichtenberger has updated the pull request incrementally with one additional commit since the last revision:

  8228363: ContextMenu.show with side=TOP does not work the first time in the presence of CSS
  
  Removed unnecessary imports.

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/383/files
  - new: https://git.openjdk.java.net/jfx/pull/383/files/b02dcb02..ba803abf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=383&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=383&range=02-03

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/383.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/383/head:pull/383

PR: https://git.openjdk.java.net/jfx/pull/383